"Our engineers have determined the root cause and taken steps to prevent a recurrence," the utility company that serves 10 million people in the nation's most populous city and nearby Westchester County explained in a statement.

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The blackout lasted from 6:47 that Saturday evening until shortly after midnight, officials said. At its height, 72,000 customers were in the dark, mainly in midtown Manhattan and parts of the Upper West Side, Con Ed said. No injuries or hospitalizations were reported.

The new report supports Con Ed's preliminary finding of problems between sensors and protective relays at the 65th Street substation. To identify the exact cause of the operational failure, it conducted "extensive testing of equipment" and "reviewed 15 years of operating data," the utility company said.

In electrical systems, a relay detects abnormal conditions and instantly sends signals to circuit breakers to open and isolate the problem, Con Ed explained.

"Out of an abundance of caution, we have taken preventive measures by isolating similar relay equipment at other substations. We will analyze and test the equipment before we put it back in service," the company said.

Outages irk New York mayor

Eight days after the Manhattan blackout, more than 50,000 customers, mostly in Brooklyn, faced a second power outage due to high usage during a stifling heat wave, Con Ed said at the time.

Speaking at that time from an emergency management command post, New York Mayor Bill de Blasio said the Brooklyn outage should have been "preventable" since it was caused by "obviously a predictable situation."

"We don't have any good answers yet why this was not prevented," he said, adding that Con Ed did not respond to his request for answers, though the outage appeared to result from a malfunctioning piece of equipment.

De Blasio had tweeted his complaints when the preliminary report on the Manhattan blackout was released: "I'm troubled that one of the few factors they initially ruled out, the 13,000 volt cable, has been determined to be the catalyst of the outage."

The New York City grid, which is one of the "most complex and technologically advanced in the world," contains multiple layers of redundancy to protect customers, Con Ed stated.

"Bottom line: this should not have happened and we're going to make sure this doesn't happen again," de Blasio said: "The city that never sleeps can't be left in the dark."